1. Field of the Invention
The present invention relates to thin films for perpendicular magnetic recording media, and more particularly related to cobalt alloy based magnetic layers.
2. Description of the Related Art
Hard disc drive areal recording density has rapidly increased ever since the industry began in the late '50s. As areal density continues to increase, the size of the magnetic domains comprising information or bits continues to decrease. As bit size continues to decrease, the ability of the media to maintain the magnetization of a bit decreases due to thermal effects that cause spontaneous de-magnetization of a bit. Thermal effects can be countered to some degree by a variety of techniques. However, even with the application of all known techniques, conventional longitudinal recording is quickly approaching a thermal limit. New approaches are needed.
Perpendicular media has greater thermal stability than longitudinal media (for similarly sized bits). See, “The feasibility of magnetic recording at 1 Terabit per square inch”, R. Wood, IEEE Trans. Magn. 36, 36 (2000). For this reason, perpendicular recording on perpendicular media may soon replace longitudinal recording.
Perpendicular magnetic recording media typically consists of a multilayer structure. The first layer is a soft magnetic underlayer. This layer is formed over a rigid substrate and provides a flux return path or flux sink. The next layer is an interlayer that is used to control the grain size and crystals structure of the magnetic layer. A magnetic layer is then formed on interlayer. This layer will have its easy magnetization axis vertically oriented. This layer, in turn, is protected by an overcoat such as carbon. The carbon layer is typically lubricated with an organic lubricant.
The magnetic layer is the layer on which the information is stored and which must be thermally stable.
Perpendicular media must not only have high thermal stability, it must have a high signal to noise ratio (“SNR”). Low noise is achieved by minimizing grain volume V. Thermal stability is increased by increasing magnetic anisotropy (“Ku”). The magnetic anisotropy (Ku) of the media should be sufficiently high so that the total magnetic anisotropy energy per grain (KuV) is large enough to overcome the thermal fluctuation effect (˜kBT) in a rigid-disk drive environment. Thus, noise and stability are trade-offs.
Hcp-structured Co-alloys, employed in conventional longitudinal recording media, have low magnetic anisotropy. For this reason they do not meet the thermal stability requirement for very high-density recording. Co3Pt-alloys, L10-phased materials (e.g. FePd, FePt, CoPt, MnAl) and rare-earth-transition-metals (e.g. Fe14Nd2B, SmCo5) are examples of alloys that do have sufficiently high Ku and therefore possible candidates for high-density, perpendicular, magnetic recording media.
Co3Pt phase alloys can exhibit a large magnetic anisotropy (Ku>2×107 erg/cc) when epitaxially grown onto single crystal substrates. The intrinsic anisotropy is associated with the chemically ordered phase of Co3Pt. The chemically ordered phase is not found in the equilibrium hcp Co—Pt phase. Nevertheless, a fully ordered Co3Pt film can reach an anisotropy as high as ˜3.1×107 erg/cc. These properties, together with an intrinsic magnetization of the pure Co3Pt phase of 1100 emu/cm3 indicate that these chemically ordered Co-alloys possess the required anisotropy and magnetization necessary for thermal stability in a tera-bit-per-square-inch magnetic recording regime.
In order to record discrete information on isolated bits, thin magnetic recording films cannot be single crystals. They must be polycrystalline, that is, be composed of separated, individual crystals also known as grains. The crystals must have their magnetic easy axis oriented perpendicular to the film plane for perpendicular recording. To reduce noise and increase thermal stability, the orientation dispersion around the film normal should also be as small as possible and the grain size distribution should be as narrow as possible. Moreover, the grain boundaries should be sufficiently wide to magnetically isolate the neighboring grains. The grain boundaries may consist of voids or non-magnetic materials.
This type of microstructure is normally obtained in the manufacture of conventional hcp Co-alloys by controlling the deposition process, modifying the interlayers and by choosing a suitable magnetic alloy composition. Compared with the high Ku materials discussed above, including chemically ordered Co3Pt, conventional hcp Co-alloys have many advantages that permit one to obtain the right equilibrium phase and to control the desirable microstructure features, i.e., orientation, grain size and its distribution, chemical segregation, composition, etc.
However, all prior chemically ordered Co3Pt films have been single-crystal films. There is no known process for making a chemically ordered Co3Pt film with all the microstructural properties described above.
Another problem presented by Co3Pt is that its saturation magnetization (“Ms”) of 1100 emu/cm3 is too high for conventional hard disc recording. High Ms gives rise to a signal that is out of dynamic range of today's reader-heads. Too much signal causes the reader to saturate.
There is a need for a chemically ordered polycrystalline thin film of Co3Pt for use in a perpendicular magnetic recording medium.